If after an extended lagering period there is fear of not having enough healthy yeast available for carbonation, how much yeast should be added back?
What is the easiest form to use, dry or liquid?
I would definitely go with dry yeast - it is cheaper, and will contribute no additional flavor to the beer.
I would go with about 10% of the amount of yeast needed for fermentation. Assuming a 11g packet of yeast, use about 1g.